The invention is related generally to centering of springs about a common longitudinal axis. In particular, the invention concerns itself with machine tool spindles which employ a central drawbar for securing a tool, where the drawbar is actuated by spring force.
It has long been known in milling machines and other tools to employ a central drawbar within the tool spindle to retain the tool which is inserted in the spindle nose. Earliest machines utilized a threaded drawbar which was inserted through the rear of the spindle and threaded into the tool shank to retain the two together. In automatic machining centers, it is well-known in the art that a drawbar is usually spring applied. That is, the drawbar is fitted with a plurality of springy collet fingers at its front end for grasping a tool stud, and the drawbar is withdrawn into the spindle to retain the stud under the force of a coaxial compression spring, such as a large helical compression spring or a plurality of nested or stacked belleville springs. The spring force in such machining centers generally is in the range of several thousand to ten thousand pounds of clamping force. The drawbar grip is released by a fluid-powered central piston applied at a tool interchange time, wherein the spring force is overcome, thus moving the drawbar to an unclamped position where the springy collet fingers will release the tool stud. The use of spring force is very desirable for tool retention, since the mechanical elements of the assembly will remain locked in the event of a machine power failure.
Several problems are inherent in the assemblies which employ spring applied drawbar clamping forces, however. The spring enlarges at its outer diameter as the spring is compressed, and, in the case of belleville springs in particular, the inner diameter becomes smaller as well. Therefore, to accommodate the diametral changes in the clamping springs, the drawbar must be made with a sufficiently reduced diameter to provide radial clearance around the spring ID, and the spindle must be machined with a clearance hole to provide radial clearance around the outer diameter of the springs. Additionally, it is well-known in the machine arts that helical compression springs will tend to buckle slightly and go off-center as they are worked. Similarly, a stack of belleville springs will tend to slip off the central axis of the drawbar as they are worked against one another, since they are small conical plates arrayed base-to-base, etc.
The movement off-center of the drawbar actuating springs does not usually present a problem in machines which have low rotational spindle speeds but, as spindle speeds are increased, vibrational forces may arise due to the off-centering mass of the springs. Modern day machining centers are being configured for rotational spindle speeds of 10,000 to 40,000 rpm and up. At these greatly increased spindle speeds, the eccentric mass problem of the springs becomes more acute.
Applicant has obviated the difficulties inherent in the state-of-the-art machining centers by providing for a spring centering means which will tend to keep the spring centered about the axis of the drawbar and thereby reduce or eliminate any vibrational problems which the springs may potentially cause.
It is therefore an object of the present invention to provide a device for centering a spring element about its longitudinal axis which comprises an axis of rotation.